The present invention relates to an image forming apparatus such as an electrophotographic apparatus, electrostatic recording apparatus or the like using a contact charging type charging device for electrically charging an image bearing member such as an electrophotographic photosensitive member, dielectric member for electrostatic recording or the like, more particularly to a cleanerless type image forming apparatus which is not provided with a cleaner exclusively for cleaning the image bearing member. The image forming apparatus such as an electrophotographic apparatus requires an electric charging step of charging the image bearing member uniformly to a predetermined potential in order to form an electrostatic latent image on the image bearing member For this purpose, a non-contact type corona charger or the like has been used as a means for the charging.
However, the corona charger produces ozone and requires such a high voltage as approx. 10 KV has to be applied between the charging device and the image bearing member.
Recently, a charging means has been proposed to avoid these problems. In such a means, a charge member is directly contacted to the image bearing member and is supplied with a voltage by which the image bearing member is charged uniformly (so-called contact charging device).
Referring first to FIG. 5, there is shown a typical contact charging device is a charging roller 2-X. S charging roller 2-x comprises an electroconductive base roller, an intermediate resistance surface layer, and the roller is rotated by t image bearing member 1 in the codirectional peripheral movements in the direction indicated by arrow f (rotational direction a). Between the roller and the image bearing member 1, a predetermined voltage is applied from a voltage source S1, so that said image bearing member 1 is electrically charged to a uniform potential.
Here, the voltage applied to the roller may be (1) a DC voltage only or (2) a DC voltage biased with an AC voltage.
(1) In the case of (1), in order to charge the image bearing member 1 to a potential of xe2x88x92600V, the applied voltage is approx. xe2x88x921300V, and in the case of (2), the applied DC voltage is xe2x88x92600V and the AC voltage is not less than 1500 Vpp.
The charging mechanism in these cases is based on the Paschen""s law, and an electric discharge phenomenon arises in a region satisfying the Paschen""s law in which the distance between the charging roller 2-X-a and the image bearing member 1 is within a predetermined range (region H in FIG. 5).
However, as will be understood from the charging mechanism, the contact charging device of this type creates the discharge which is the same as with the corona charger within a fine space region H, and therefore, the ozone is produced although the amount of ozone production is remarkably smaller than with the corona charger. The ozone produces nitrogen oxide, and if it is deposited on the image bearing member 1, an image defect is produced due to the low resistance of the deposited matter.
This injection charging process system is proposed in U.S. Pat. Nos. 6,134,407 or 6,081,681 and 6,128,456 in which is free of such a problem of ozone generation, and therefore, the voltage applied to the charging device can be further reduced.
The feature of the charging process is that surface potential of the charged image bearing member is substantially the same as the voltage applied to the charging device. This system does not use the electric discharge phenomenon, and charge injection occurs into the image bearing member by the transfer of electric charges between the surface of the image bearing member and the charge member contacted thereto.
FIG. 6 is a schematic view of a major part of the injection charging device 2. A charging sponge roller 2-A (charging roller) of an electroconductive sponge carries electroconductive particles Z deposited on its surface and rotates in such a direction that surface there is moved counterdirectionally (b) relative to the peripheral moving direction (a) of the image bearing member 1 at a nip C formed between t image bearing member 1 and the charging sponge roller, while injecting charge into the image bearing member 1 from the charging sponge roller 2-A. By this, the image bearing member 1 is charged to a potential substantially equal to the potential of the charging sponge roller 2-A.
The electroconductive particle are fine electroconductive particles (charging-promotion particle) for assisting the charging. The electroconductive particles are metal oxide fine particles of electroconductive zinc oxide or the like having a volume resistivity of not more than 1xc3x971012 xcexa9.cm, preferably not more than 1xc3x971010 xcexa9.cm, with or without electroconductive inorganic fine particles, organic material mixed therewith.
In this system, the charging sponge roller 2-A is supplied with a DC voltage of xe2x88x92600V from a voltage source S1. Therefore, the surface potential of the image bearing member 1 tends to become the same potential at the portions where the charging sponge roller 2-A and the electroconductive particle Z are contacted. At this time, if the charge is injected into the image bearing member 1 from the charging sponge roller 2-A side beyond an energy barrier on the surface of the image bearing member 1, the image bearing member 1 is electrically charged. If not, or if the charge moves back from the image bearing member 1 to the charging sponge roller 2-A side at positions where the charging sponge roller 2-A and the image bearing member 1 are apart from each other, the image bearing member 1 is not charged. This phenomenon is dependent on the energy barrier of the surface of the image bearing member 1 and/or the charge retention power. On the other hand, when it is taken as a competitive reaction, a frequency of chance of contact between the charging sponge roller 2-A side and the image bearing member 1 is important. In order to increase the frequency, electroconductive particle Z having small particle sizes are deposited on the surface of the charging sponge roller 2-A so as to increase the injection sites in the contact portion C between the image bearing member 1 and the charging sponge roller 2-A, and in addition, the charging sponge roller 2-A is rotated in the peripheral counterdirectional direction so as to increase the relative speed between the image bearing member 1 and the charging sponge roller 2-A, thus increasing the number of contact to the image bearing member 1 in the injection sites per unit time.
In this manner, the charging sponge roller 2-A and the electroconductive particle Z which establish injection sites of the charge to the image bearing member 1 are contacted to the image bearing member 1 at high opportunity, so that surface potential of the image bearing member 1 becomes substantially the same potential, that is, xe2x88x92600V applied to the charging sponge roller 2-A. Microscopically, uniform charging is accomplished.
FIG. 7 is a schematic view of an example of a transfer type electrophotographic apparatus of a cleaner-less system type wherein the charging means for the image bearing member 1 is an injection charging device 2 using the electroconductive particles Z as described above, and no cleaner exclusively for cleaning the image bearing member 1 is used.
Designated by reference numeral 1 is an electrophotographic photosensitive member of a rotatable drum type (image bearing member), wherein rotated at a predetermined peripheral speed in the clockwise direction indicated by an arrow a. Designated by 2-A is a charging sponge roller, which is contacted to the image bearing member 1 with a predetermined urging force to provide a contact portion (charging nip) C having a predetermined width. On the outer surface of the charging sponge roller 2, charging sponge rollers 2 are deposited beforehand. The charging sponge roller 2 is rotated in the clockwise direction indicated by an arrow b, by which the charging sponge roller 2 is rotated counterdirectionally with respect to the peripheral movement of the image bearing member 1 at the contact portion C relative to the image bearing member 1, and the charging sponge roller 2 is supplied with a predetermined charging bias from the voltage source S1, by which the outer surface of the image bearing member 1 is uniformly charged by charge injection to a predetermined potential of a predetermined polarity.
The surface of the image bearing member 1 thus uniformly charged is exposed to image light L by unshown exposure means (digital scanning exposure device such as a laser beam scanner or the like, image projecting device for projecting an image of an original document, so that electrostatic latent image is formed corresponding to the exposure image pattern on the uniformly charged surface of the image bearing member 1.
Then, the electrostatic latent image is visualized into a developed image (toner image) by a developing sleeve 3-a in a jumping developing device 3 of a non-contact type. Designated by S2 is a voltage source for applying a predetermined developing bias voltage to the developing sleeve 3-a. 
Then, the developed image is transfer, at a transfer portion where a transfer roller 5-a of a transferring device 5 is contacted to the image bearing member 1 onto a transfer sheet P (recording material sheet fed from an unshown sheet feeder at predetermined controlled timing. Designated by S3 is a voltage source for applying a predetermined transfer bias to the transferring device 5.
The recording material P is separated from the image bearing member 1 after receiving the developed image at the transfer portion and is introduced into an unshown fixing device, where the image is fixed. Then, the recording material P is discharged as a print (or copy).
After the separation of the recording material, the residual developer remaining on the surface of the image bearing member 1 is carried back to the developing zone by way of the continuing rotation of the image bearing member 1, and is collected into the developing device 3 (simultaneous development and cleaning).
Here, the electroconductive particle retaining force of the charging sponge roller 2-A is not so strong, and therefore, a system for stably supplying the electroconductive particles Z to the charging sponge roller 2-A is provided. In this system, the electroconductive particles Z are mixed in the developer T in the developing container 3-d of the developing device 3, so that electroconductive particles Z are supplied to the charging sponge roller 2-A by way of the image bearing member 1, that is, supplied to the image bearing member 1 through the non-contact jumping developing device 3 and then carried on the image bearing member 1 to the position of the charging sponge roller 2-A.
In the non-contact jumping developing device 3, the electroconductive particles Z are supplied when the developer T is supplied to the image bearing member 1 for development. The charging polarity of the electroconductive particles Z is selected so as to be opposite the regular charging polarity of the developer T, so that they are not transferred onto the transfer sheet P by the transfer roller 5-a but remains on the image bearing member 1. Then, they are collected by the sponge roller 2-A which is rotating counterdirectionally relative to peripheral movement of the image bearing member 1.
This system is a so-called cleaner-less system in which no cleaning process for collecting the developer T between the charging step and the transfer step in the image formation process. If an attempt is made to implement the cleaner-less system of this Examiner in a conventional type contact charging device or the like described above, the residual developer T after the transfer step appears as it is in the next formed image. In this example, however, the charging sponge roller 2-A of the elastic member is rotated in the counterdirectional peripheral movement relative to the image bearing member 1, the residual developer T is scraped, and therefore, no influence is imparted to the next image. Most of the developer T deposited on the charging sponge roller 2-A is discharged to the image bearing member 1 at a relatively early stage, and thereafter is collection into the developing device 3 when it passes through the developing zone. Therefore, the cleanerless electrophotographic process is accomplished.
Referring to FIGS. 8 and 9, the description will be made in detail as to the effect of the counterdirectional rotation of the charging sponge roller 2-A in the cleaner-less system, will be described. (a) if the charging sponge roller 2-A is rotated in the codirectional peripheral movement, the developer T on the image bearing member 1, is deposited to the charging sponge roller 2-A during the passage thereof through the contact portion C, and the amount of the developer T decreases. But, the previous image pattern still remains as it is, and would result in the image defect in the next image. (b) if the charging sponge roller 2-A is not rotated, the developer T, as shown in FIG. 8, stagnates at the inlet portion J of the contact portion C. When the amount of the stagnated developer reaches a predetermined amount, the developer T enters the contact portion C, and therefore, the charging property lowers.
On the other hand, in the case of the counterdirectional rotation, the developer T, as shown in FIG. 9, is scrapped off the image bearing member 1, and therefore, the developer T deposited on the image bearing member 1 before the passing through the contact portion C relative to the charging sponge roller 2-A is not present on the image bearing member 1 after passing through the upper. The developer T scraped off the image bearing member 1 is not directly deposited onto the charging sponge roller 2-A surface, but is once stagnated at the inlet portion J of the contact portion C.
As indicated by an arrow j in the Figure, behavior of the developer is moved to the charging sponge roller 2-A while making a circulating motion by the feeding force provided by the surface of the image bearing member 1 and by the feeding force provided by the surface of the charging sponge roller 2-A, so that said image pattern does not remain as it is on the charging sponge roller 2-A.
The amount of the developer T moving to the charging sponge roller 2-A is different depending on the cases, for example, the amount of the residual developer T is large after development of a black image image so that large amount of the developer T comes to the J portion, or the latent image potential a position J changes. However, in normal use, it does not occur that large amount of the toner moves at once to the charging sponge roller 2-A. For this reason, these situations does not result in the improper charging or the image defect because of the developer T movement from the charging sponge roller 2-A to the image bearing member 1.
Most of the developer T is electrically charged to the positive polarity by the transfer step, but is charged to the negative polarity relatively quickly because of the triboelectric charge in the J portion, the negative bias voltage application in the charging step.
Thereafter, most of the developer T is carried on the charging sponge roller 2-A, but is moved to the image bearing member 1 at the outlet portion K of the contact portion C between the image bearing member 1 and the charging sponge roller 2-A. As described in the foregoing, the developer T on the charging sponge roller 2-A is charged to the negative polarity. In addition, when the comparison is made between the potential of the surface of the image bearing member 1 at said K portion and a potential of the charging sponge roller 2-A, the potential of the charging sponge roller 2-A is relatively at the negative side even if it may be slightly so.
The developer T having moved to the image bearing member 1 before it enters the contact portion C between the image bearing member 1 and the charging sponge roller 2-A, is fed to the developing device away from the contact portion C by the feeding force of the image bearing member 1. For this reason, most of the developer T does not enter the contact portion C, and the deterioration of the charging property does not occur.
As compared with the behavior of the electroconductive particle Z, the developer T does not easily enter the contact portion C because or the particle sizes thereof as well as the charging polarity, and therefore, only the electroconductive particles Z tend to be present at the portion. Therefore, satisfactory charging property can be provided.
(I) The state at the time of start of the image forming step is particularly considered in implementing an image forming process using the injection charging device 2, when the start of the rotation of the image bearing member 1 is earlier than the start of the rotation of the charging sponge roller 2-A, the situation is the same as with the case in which the charging sponge roller 2-A does not rotate (b). In normal use, the residual developer T does not present on the image bearing member 1 at the initial start. If, however, the amount of the developer T discharged on the image bearing member 1 from the transfer roller 5-a during the cleaning process for the transfer roller 5-a in the transferring device 5 after the image formations, is large, or if a transfer sheet P is jammed, a large amount of the developer T stagnates at the inlet portion J of the contact portion C. If this occurs, the charging property is temporarily deteriorated as described in the foregoing (b), and in addition, at the contact portion C between the image bearing member 1 and the charging sponge roller 2-A, the developer T might be embedded into the charging sponge roller 2-A, and therefore, developer T is not easily separated from the charging sponge roller 2-A with the result of continuing local improper charging. When the rotation of the charging sponge roller 2-A starts, the developer T stagnated at the J portion is fed at once to the charging sponge roller 2-A, and the resistance change on the charging sponge roller 2-A occurs with the result of change in the charged potential which leads to charging non-uniformity. Because the large amount of the developer T is supplied to the image bearing member 1, image defect will be produced in some cases.
The case that few amount of the developer T is present on the image bearing member 1 or the J portion at the time of the start will be considered. When a large amount of the electroconductive particles Z for increasing the charging performance are present thereat, they are concentrated at the J portion, and electroconductive particles Z having small particle sizes are embedded into the charging sponge roller 2-A by the feeding force provided by the rotation of the image bearing member 1. Only the portions of the charging sponge roller 2-A that is supplied with a larger amount of the electroconductive particles Z with the rotation of the charging sponge roller 2-A, has a low resistance, that is, a high charging performance, with the result of high latent image potential portions in the form of stripes and therefore non-uniform image.
In order to solve the problem, the amounts of the developer T and the electroconductive particles Z at the J portion at the time of the start of the image forming process is made always stabilize. It would be considered that developer T collecting operation in the developing process is continued while performing the charging step without performing the cleaning process for the transfer roller 5-a or the image forming process. In such a case, the post-rotation step which is executed after the image forming operation requires a long time, in order to solve the problem, the amounts of the developer T and the electroconductive particles Z at the J portion at the time of the start at image forming process is made always stabilize. It would be considered that developer T collecting operation in the developing process is continued while performing the charging step without performing the cleaning process for the transfer roller 5-a or the image forming process. In such a case, the post-rotation step which is executed after the image forming operation requires a long time.
Since the charging sponge roller 2-A is made of elastic material, the charging sponge roller 2-A easily deforms at the contact portion C relative to the image bearing member 1 if the image forming apparatus is kept in a non-operative state for a long term. In the normal use, the change in the charging state resulting from the change of the configuration is not as significant as being the influential to the resultant image. However, the tendency toward said improper charging is overlaid on the deformed portion, the improper charging may easily occurs at the deformed portion.
If a relatively large amount of the developer T exists at the J portion after the end of the previous image forming process, or the like, a large amount of the developer T stagnates at the K portion after one full-turn of the charging sponge roller 2-A before the rotation of the image bearing member 1 as shown in FIG. 10. If only the rotation of the charging sponge roller 2-A continues, the developer T at the K portion enters the contact portion C between the image bearing member 1 and the charging sponge roller 2-A by the feeding force of the charging sponge roller 2-A, with the result of tendency of the improper charging.
The case that charging step is carried out with the system (1) using an electroconductive brush roller 2-B, as shown in FIG. 11, supplied with a DC voltage, will be considered. If the image bearing member 1 is rotated without rotation of the electroconductive brush roller 2-B, a large amount of the developer T may be carried to the contact portion C between the image bearing member 1 and brush roller 2-B, and the developer may enter the contact portion C of the brush roller 2-B. The developer is not easily separated from the inside of the brush, and no electroconductive particle as are supplied from the outside, and therefore, the resistance of the brush roller only at such a position continues to be high even if the brush roller rotates. This would result in the non-uniform charging and therefore non-uniform image. (II) The ending stage of the image forming process executed using the injection charging device 2 will be considered. When the rotation of the charging sponge roller 2-A is stopped earlier than the stop of rotation of the image bearing member 1, the situation is the same as with (b) described above, that is, the case that charging sponge roller 2-A does not rotate. In this case, when a cleaning process for the transfer roller 5-a of the transferring device 5 is carried out after a series of image formations process, and the amount of the developer T discharged from transfer roller 5-a onto the image bearing member 1 is large, a large amount of the developer T stagnates at the inlet portion J of the contact portion C, as described hereinbefore.
When the transfer sheet P jam or the like occurs in the image forming apparatus, recovery sequential operations are normally carried out, including cleaning sequential operations for removing the large amount of the developer T remaining on the image bearing member. However, it is difficult to remove the developer T from the image bearing member 1 to reduce the amount of the developer down to a normal level in a short period of time. Therefore, in many cases, the amount of the developer T is larger than in the normal state even if no image defect results.
Therefore, in this case, too, the similar problems arise when the stop of rotation of the charging sponge roller 2-A is earlier than the stopover rotation of the image bearing member 1.
At the contact portion C between the image bearing member 1 and the charging sponge roller 2-A, the developer T is embedded into the charging sponge roller 2-A, and developer T is not easily separated from the charging sponge roller 2-A there. In the subsequent image forming operations, local improper charging may occur there.
When the rotation of the image bearing member 1 starts with the state, a large amount of the developer is deposited to the portion which took the position J at the time of start of the rotation as shown in FIG. 12, with a result of change of the charged potential at that portion and therefore non-uniform charging. Or, a large amount of the developer T is altogether supplied to the image bearing member 1 with the result of image defect.
That case that few amount of the developer T is present at the J portion or on the image bearing member 1 will be considered. When a large amount of the electroconductive particles Z for enhancing the charging property is present at that portion, a large amount of the electroconductive particles Z are collected at the J portion, and electroconductive particles Z having small particle sizes are embedded into the charging sponge roller 2-A by the feeding force provided by the rotation of the image bearing member 1. Only the portions of the charging sponge roller 2-A that is supplied with a larger amount of the electroconductive particles Z with the rotation of the charging sponge roller 2-A, has a low resistance, that is, a high charging performance, with the result of high latent image potential portions in the form of stripes and therefore non-uniform image.
In the case of use of elastic material for the charging member, as shown in FIG. 13, the deformation and/or the contact pressure of the charging sponge roller 2-A are different between when the image bearing member 1 is rotated ((a) of FIG. 13) and when it is not rotated ((b) of FIG. 13). Generally, the area at the contact portion C is larger when the image bearing member 1 is not rotated. For this reason, there is a portion which is the J portion during rotation but is the contact portion C after the stop.
Therefore, when a large amount of the developer T and the electroconductive particles Z are supplied to the J portion, and the portion becomes a contact portion C between the image bearing member 1 and the charging sponge roller 2-A, the particle are embedded into the charging sponge roller 2-A. If such a state is kept for a long term, the contact pressure thereat becomes high with the result of the particles are embedded in a further extended and therefore further difficulty or discharge, that is, persistent non-uniformity in the charging.
Furthermore, if the apparatus is not operated for a long term under a high temperature ambience, a large amount of low resistance electroconductive particles at the contact portion C causes memory effect of charge on image bearing member 1, by which the sensitivity of the image bearing member 1 is locally different with the result of image defect.
In addition, the charging sponge roller 2-A is made of elastic material, and therefore, if the image forming apparatus is not operated for a long term, the charging sponge roller 2-A may deform at the contact portion C between the image bearing member 1. In the normal use, the change in the charging state resulting from the change of the configuration is not as significant as being the influential to the resultant image. However, the tendency toward said improper charging is overlaid on the deformed portion, the improper charging may easily occurs at the deformed portion.
In order to solve the problem, the amounts of the developer T and the electroconductive particles Z at the J portion at the time of the start of the image forming process is wade always stabilize. It would be considered that developer T collecting operation in the developing process is continued while performing the charging step after performing the cleaning process for the transfer roller 5-a after the image forming process. In such a case, however, the post-rotation step which is executed after the image forming operation requires a long time.
If a relatively large amount of the developer T exists at the J portion after the end of the previous image forming process, or the like, a large amount of the developer T stagnates at the K portion after one full-turn of the charging sponge roller 2-A after stop of the image bearing member 1 as shown in FIG. 10. If only the rotation of the charging sponge roller 2-A continues, the developer T at the K portion enters the contact portion C between the image bearing member 1 and the charging sponge roller 2-A by the feeding force of the charging sponge roller 2-A, with the result of tendency of the improper charging.
The case that charging step is carried out with the system (1) using an electroconductive brush roller 2-B, as shown in FIG. 11, supplied with a DC voltage will be considered. If the image bearing member 1 is rotated without rotation of the electroconductive brush roller 2-B, a large amount of the developer T may be carried to the contact portion C between the image bearing member 1 and brush roller 2-B, and the developer may enter the contact portion C of the brush roller 2-B. The developer is not easily separated from the inside of the brush, and no electroconductive particle as are supplied from the outside, and therefore, the resistance of the brush roller only at such a position continues to be high even if the brush roller rotates. This would result in the non-uniform charging and therefore non-uniform image.
Accordingly, it is a principal object of the present invention to provide an image forming apparatus in which even if rotation of an image bearing member starts when a relatively large amount of a developer is present on the image bearing member or when the rotation stops, the non-uniform electric charging of the image bearing member attributable to the local deposition of the developer or the like on the charging member can be prevented.
It is another object of the present invention to provide an image forming apparatus in which passage of a large amount of the developer into a contact portion between an image bearing member and a charging member. It is a further object of the present invention to provide an image forming apparatus in which an image pattern of an image formation process appears in the next image formation.
It is a further object of the present invention to provide an image forming apparatus suitable for a cleanerless type in which the image forming apparatus is not provided with a cleaner exclusively for the cleaning of the image bearing member.
It is a further object of the present invention to provide an image forming apparatus in which an image bearing member is electrically charged using electroconductive particles at a contact portion between the image bearing member and a charging member, wherein the non-uniform electric charging of the image bearing member attributable to the local deposition of the developer or the like on the charging member can be prevented.
It is a further object of the present invention to provide an image forming apparatus in which local embedding of the developer into the surface or the charging member and the resultant deterioration of the uniformity in electrical charging.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.